Field of the Invention
The present invention relates generally to thin fiber-reinforced plastic composite plates widely used, for example, for base materials of electronic parts, aircraft parts, covers or containers of various appliances, etc., in various fields. In particular, the invention relates to a method of molding such a thin fiber-reinforced plastic composite plate.
Background of the Invention
Generally, it is not easy to obtain fiber-reinforced plastic moldings having a very thin thickness, for example, of 2 mm or less. In an injection molding method which is most used at present, a fiber-reinforced plastic material must be extremely thin, for example, not thicker than 2 mm so that the fibers are allowed to move easily together with the fused plastic. Further, the productivity has not been able to reach a high level, unlike the case of manufacturing metal thin plates in which high speed mass production can be effected.
In the case where fibers, particularly long fibers, are included in the plastic molding, it is not easy to control the movement of the fibers in other methods for molding plastics such as blow molding, compression molding, or the like. As an improved measure, however, there is a hand lay-up method in which layers of glass-fiber cloth are stacked one on another while being impregnated with resin. As other measures, fiber-reinforced plastics molding methods employing a machine such as a metal-working press are practically used. Those methods include, for example, a sheet-mold compound method (SMC method) and a heat molding method for fiber-reinforced thermoplastic plastics (stampable sheet).
On the other hand, the pressing of the above-mentioned stampable sheet has recently been anticipated as a method in which thin plate moldings can be obtained by a widely used stamping method for thin metal plates and by using a machine having a high speed mass production property. According to this method, a 3-10 mm thick sheet of Nylon or polypropylene containing 30-70% of glass fibers or the like is pressed while being heated. The pressed sheet is then cooled to thereby obtain a shallow container or other body having a desired shaped. The procedure, however, is not suitable for a plate thinner than 3 mm. This is because obtaining such an extremely thin plate of fiber-reinforced plastics is considered difficult from the industrial viewpoint as described above.
Further, in carrying out press-working on the resin plate of the kind as described above, it is extremely difficult to obtain a complicated molding with substantial deformation of the resin plate. For example, assume that two kinds of thin plates, one being very thin and the other being thick, are simply bent by using presses having the same apex angle and the same radius, as shown in FIGS. 1a and 1b, respectively. In the case of FIG. 1a where the plate is very thin, a little quantity of extensionsal deformation is generated on the outside of the thin plate and the inside of the thin plate is well fitted to the shape of a tool. However, in the case of a thick plate, as shown in FIG. 1b, the quantity of outside extensional deformation is so large that a crack h is caused, and the fibers may be broken to lose their reinforcing effect because the fibers per se are generally rigid and cannot be stressed. Also, a bending stress remains in the fibers to prevent straightening the inside of the thin plate even if the fibers could be bent. That is, it must be said that accurate working is very difficult to perform on the fiber-reinforced thermoplastic resin plate of this kind.